Recovery of precious metals

ABSTRACT

Recovery of precious metals such as gold and silver from aqueous cyanide solutions thereof, by contact with a reagent containing a guanidine functionality. The guanidine reagent extracts the precious metal from the aqueous solution which is subsequently stripped from the guanidine reagent and recovered by conventional methods. Certain novel guanidine compounds suitable for extracting gold and silver are disclosed.

This application is a division of application Ser. No. 06/819,778, filedJan. 16, 1989, U.S. Pat. No. 4,814,007, issued Mar. 21, 1989.

FIELD OF THE INVENTION

This invention relates to the recovery of precious metals such as goldand silver and in particular to the recovery of gold from aqueouscyanide solutions thereof. The recovery is achieved by contact of theaqueous cyanide solution containing the precious metals, particularlygold, with a reagent containing a guanidine functionality. The guanidinereagent extracts the gold from the aqueous solution and is subsequentlystripped from the guanidine reagent and recovered by conventionalmethods. The invention also relates to certain novel guanidine compoundswhich are suitable for extracting gold from cyanide solutions.

BACKGROUND OF THE INVENTION

Gold occurs primarily as the native metal, alloyed with silver or othermetals or as tellurides. It is commonly associated with the sulfides ofiron, silver, arsenic, antimony and copper. Silver occurs as finelydisemminated metal in rocks of hydrothermal origin as silver chloride,sulfide or tellurides and as complex sulfides with antimony and arsenic.Historical practice with ores containing native metal involve crushing,concentration of the gold or silver by gravity separation and recoveryby amalgamation with mercury. Environmental concerns have resulted inabandonment of this process in most cases. Currently there are two majorprocesses for recovery of gold and/or silver. The most widely acceptedprocesses today involve leaching with caustic cyanide solution coupledwith recovery of the metal values by concentration with zinc dust(Merrill-Crowe) or concentration of the gold and silver cyanidecomplexes by absorption on charcoal followed by electrowinning (carbonabsorption scheme) also referred to as Carbon in Pulp (CIP). Anotherprocess recently practiced in the Soviet Union is one in whichquaternary amine ion exchange resins are employed as a replacement ofcharcoal in the carbon absorption scheme.

In a recent publication "Selectivity Considerations in the AmineExtraction of Gold from Alkaline Cyanide Solutions" by M. A. Mooiman andJ.D. Miller in "Minerals and Metallurgical Processing", August 1984Pages 153-157, there is described the use of primary, secondary andtertiary amines to which have been added certain Lewis base modifierssuch as phosphorus oxides and phosphate esters for the extraction ofgold from alkaline cyanide solutions.

The leach liquors containing the gold are achieved by leaching withcyanide solutions through either the dump or heap leaching techniques.In heap leaching, the ore is placed on specially prepared imperviouspads and a leaching solution is then applied to the top of the heap andthen allowed to percolate down through the heap. The solution containingthe dissolved metal values eventually collects along the impervious padsand flows along it to a collection basin. From the collection basin, thesolution is pumped to the recovery plant. Dump bleaching is similar toheap leaching in which old mine wastes dumps which have sufficient metalvalue to justify processing are leached in place. Successful dumpleaching requires careful control of leach solutions to prevent groundwater contamination. Heap leaching is replaced by vat leaching in areaswith harsh winters, wet tropical climates or limited oxygenavailability. In vat leaching the crushed ore is placed in a vat whichis then flooded with leaching solution. In any of the leaching methods,a cyanide leach solution is obtained from which the metal values arerecovered. While cyanide leaching is in general use today, it isrelatively slow and has toxic disadvantages. Other leaching solutionsare being considered in which thiourea, i.e. acido thiourea is employed,and/or thiocyanates. However, cyanide solutions appear to be the reagentof choice as a primary lixiviant for gold.

In the CIP process, coconut shell activated carbon is necessary, whichis in short supply and expensive. In the Merril-Crowe process zinc dustis used to precipitate gold from the clarified cyanide solution, but thecost of this process to separate gold from leach liquors is similar tothat of the CIP method.

Different amine functionalities have been considered in the past in boththe liquid/liquid extraction and liquid/solid extraction of gold. Forliquid/solid extraction auricyanide is too strongly bound with thequaternary amines of the resins, so that stripping is difficult andrequires special treatment. In addition, no selectivity of metal cyanidecomplexes and leach liqors is shown. Resins with weaker basic aminefunctionalities cannot perform well in the pH range (10-11), the pH ofthe common leach liqors. For liquid/liquid extraction such as the workof Mooiman and Miller, organophosphorus modifiers, i.e.trialkylphosphates are required to increase the amine basicity in orderto permit efficient extraction of the gold materials. These materialsmust be used in large amounts. The systems still do not reach or meetthe pH criteria of leach liqors.

BRIEF SUMMARY OF THE INVENTION

It has now been discovered that certain compounds having a guanidinefunctionality provide a method for extraction and recovery of preciousmetals such as gold and silver. The guanidine functional reagentsprovide both a liquid/solid and a liquid/liquid system which is usefulat the pH levels of the cyanide leach solutions commonly employed inprocesses for recovering gold.

In a liquid/liquid extraction method, the reagent must be soluble in anorganic solvent which is immiscible in relation to the aqueous cyanideleach solution. Thus, the guanidine reagent is dissolved in the organicsolvent, which is then brought in contact with the aqueous cyanidesolution containing the desired metal values. The guanidine reagentextracts the gold and/or silver metals from the cyanide leach solutionwhich are now found in the organic phase which is immiscible with theaqueous phase. After separation of the organic phase from the aqueousphase, the organic phase containing the desired metal values are thenstripped by contact with an aqueous caustic solution which strips themetal values from the organic phase. The metal values now in a moreconcentrated aqueous solution are then recovered in conventional methodssuch as those used in the carbon absorption method throughelectro-winning.

In the liquid/solid extraction method, a guanidine reagent is firstincorporated into a solid ion exchange carrier. Recovery of the goldfrom the cyanide solution is accomplished by contacting the cyanidesolution with the ion exchange reagent carrier containing the guanidinefunctionality, at which point the metals are extracted from the aqueouscyanide solution onto the ion exchange carrier containing the guanidinereagent. The metal barren aqueous solution is then separated from thecarrier containing the guanidine. The metal values are then strippedfrom the ion exchange carrier containing the guanidine functionality andrecovered in the same manner as in the liquid/liquid extraction method.

Accordingly, the present invention is directed to a process for therecovery of precious metals such as gold or silver from an aqueoussolution containing such metal values comprising

(1) contacting the aqueous solution with a compound containing afunctional guanidine group to extract at least a portion of the preciousmetal values from the aqueous solution

(2) separating the resultant metal-barren solution from the guanidinecompound, and

(3) recovering the precious metals from the guanidine compound.

The present invention is further directed to certain novel guanidinecompounds and novel ion exchange resins carrying a guanidinefunctionality. By guanidine functionality is meant those compounds,reagents or ion exchange resins containing the functional group:##STR1## In regard to the ion exchange resins the group is bonded bychemical reaction to the resin through any one of the N atoms. Theremaining bonds of the nitrogen atom are filled by hydrogen, aliphaticor aromatic hydrocarbon groups or cyclic (including heterocyclic groupscontaining nitrogen atoms), straight or branched chain, saturated andunsaturated, as will be discussed in more detail in the description tofollow. Aspects and advantages of the present invention will be apparentto those skilled in the art upon consideration of the following detaileddescription thereof.

DETAILED DESCRIPTION AND PREFERRED EMBODIMENT

According to the present invention, reagents useful in the recovery ofprecious metal values such as gold and copper have been found whichcontain a guanidine functionality. These reagents possess the desirableproperties necessary for successful application as an ion exchangereagent. These guanidine functional containing compounds meet a numberof criteria which make them useful for this purpose. In the firstinstance, the guanidine compounds complex with or react with theprecious metal and do so in relatively fast order, which avoids havingto use large holding tanks or reaction vessels. The compounds exhibit aselectivity at designated pH ranges. The reagents exhibit satisfactorysolubility in the essentially water-immiscible organic solvents used inliquid/liquid extraction systems to form the organic phases, or arecapable of being rendered soluble to a sufficient extent in such organicsolvent through the use of solubility modifiers. In addition, theguanidine reagent metal complexing is reversible in that the metal canbe readily stripped from the organic phase. Further, compounds may bechemically reacted with ion exchange resins to provide a resincontaining the guanidine functionality which is useful in liquid/solidextraction systems.

The liquid/liquid process of the invention is a liquid ion exchangeprocess in which a water-insoluble guanidine compound is dissolved in anessentially water-immiscible liquid hydrocarbon solvent and theresulting solution is contacted with a metal-containing aqueous phase toextract a portion of the metal values into the organic phase. The phasesare then separated and metal values are stripped from the organic phaseby the use of an aqueous stripping medium.

A wide variety of essentially water-immiscible liquid hydrocarbonsolvents can be used in the metal recovery process of the presentinvention. These include: aliphatic and aromatic hydrocarbons such askerosenes, benzene, toluene, xylene and the like. A choice of theessentially water-immiscible liquid hydrocarbon solvents for particularcommercial operations will depend on a number of factors, including thedesign of the solvent extraction plant (i.e. mixer-settler units,Podbielniak extractors, etc.), the value of the metal being recovered,and the like. The process of the present invention finds particular usein the extraction recovery of the precious metals such as gold and/orsilver. The preferred solvents for use in these precious metal recoveryprocesses of the present invention are the aliphatic and aromatichydrocarbons having flash points of 150° F. and higher and solubilitiesin water of less than 0.1% by weight. The solvents are also essentiallychemically inert. Representative commercially available solvents areKermac 470B (an aliphatic kerosene available from Kerr-McGee--flashpoint 175° F.), Chevron ion exchange solvent (available from StandardOil of California--flash point 195° F.), Escaid 100 and 110 (availablefrom Exxon-Europe--flash point 180° F.), Norpar 12 (available fromExxon-USA--flash point 160° F.), Conoco-C1214 (available fromConoco--flash point 60° F.), Aromatic 150 (an aromatic keroseneavailable from Exxon-USA--flash point 150° F.), the various otherkerosenes and petroleum fractions available from other oil companies. Inthe process of the present invention, the organic solvent solutions willpreferably contain from about 0.02 to 20% by weight of the guanidinecompound and even more preferably from about 0.1-5% by weight thereof.Additionally, volume ratios of the organic:aqueous phase vary widelysince the contacting of any quantity of the guanidine solution with themetal containing aqueous phase will result in extraction of metal valuesinto the organic phase. However, for commercial practicality, theorganic:aqueous phase ratios are preferably in the range of about 50:1to 1:50. It is desirable to maintain an effective 0 to A ratio of about1:1 in the mixer by recycle of one of the streams. For practicalpurposes the extracting and stripping are normally conducted at ambienttemperatures and pressures, although higher and/or lower temperaturesand/or pressures are entirely operable. Most advantageously, the entireprocess can be carried out continuously with the stripped organicsolvent solution being recycled for contacting further quantities of theprecious metal-containing cyanide solutions. As indicated, in aliquid/liquid extraction process the guanidine reagent must be solublein the organic water-immiscible solvent to the extent of about 0.02% byweight, or capable of being soluble to such extent through the use of asolubility modifier substance. Such solubility modifiers suitable foruse in the present invention include longchain (C₆ -C₂₀) aliphaticalcohols such as n-hexanol, n-2-ethylhexanol, isodecanol, dodecanol,tridecanol, hexadecanol and octadecanol; longchain alkyl phenols such asheptylphenol, octylphenol, nonylphenol and docecylphenol; andorgano-phosphorus compounds such as tri-lower alkyl (C₄ -C₈) phosphates,especially tributyl phosphate and tri(2-ethylhexyl) phosphate.

The extraction of the precious metals from their aqueous solutiondepends on a number of factors including, for example, the concentrationof the metal ion, the particular anions present, and the pH of theaqueous solutions and the concentration of and the particular guanidineused in the organic phase. Thus, for each aqueous metal solution andreagent solution of guanidine, there will be a preferred or optimum setof extraction conditions and those skilled in the art based on theinformation given herein, especially in respect of the examples tofollow, will be able with a limited number of trial runs to determinesuch preferred or optimum conditions for the specific system underconsideration. This is equally true of the stripping operations. Bystripping is meant that at least a portion of the metal values in theloaded organic phase are transferred to the aqueous stripping medium.The metal values are then desirably recovered from the aqueous strippingmedium by conventional techniques, preferably electrolysis. The loadedorganic:aqueous stripping phase ratios can also vary widely. However,the overall object of the process is to provide a metal containingstripping solution of known composition and concentration suitable forthe conventional recovery techniques such as by electrolysis. Thus,normally the metal will preferably be present in higher concentrationsin the aqueous stripping medium than in the starting metal-containingsolution. In this regard the starting aqueous metal-containing solutionswill contain 1 to 5 ppm of gold, 1 to 2 ppm of silver and 5 to 10 ppm ofcopper plus traces of other metals. A heap leach liquor will average 0.5to 2 ppm gold, 0.5 to 2 ppm silver and 5 to 100 ppm copper plus othermetals. The concentrations of gold in the aqueous strip solutions fromwhich the gold will be recovered will be anywhere from about 50 to 1000ppm. This will largely depend on the stripping solutions employed andthe efficiency thereof. In the stripping step, the loadedorganic:aqueous stripping medium phase ratio will preferably be in therange of about 1:1 to 20:1. The aqueous stripping solutions for use inthe present invention will generally be basic stripping solutions havingpH in excess of 11.0. The stripping reagent preferably employed iscaustic sodium hydroxide solution having a pH above 11, generally 12 orabove. After removal of the metal from the aqueous stripping solution byconventional techniques, the caustic aqueous solution was recycled.

The foregoing description has dealt with the liquid/liquid extractionsystems. As earlier indicated, liquid/solid systems can be employed, inwhich a guanidine reagent is incorporated into an ion exchange resin bychemically bonding the guanidine functionality to the resin backbone. Inthis regard, the term "extracting" used herein is to be understood asincluding both liquid and solid means for selectively removing andotherwise separating the precious metal values. As the ion exchangeresin containing the guanidine functionality will be used to treat orcontact a gold-containing aqueous solution, the ion exchange resin mustbe one which is water-insoluble. Upon contact of the aqueous cyanidesolution containing the precious metals, the precious metals areselectively absorbed by the guanidine reagent on the the ion exchangeresin. The metal values are then eluted from the ion exchange resin bycontact with the sodium hydroxide solution such as the strippingsolution mentioned earlier above. The techniques employed in theproduction of water-insoluble ion exchange resins employed in theprocess of the present invention are well-known to those skilled in theart, and especially, to those skilled in the art of polymerizingmonomers to produce polymeric compositions useful as ion exchangeresins. In the present invention, the preferred ion exchange resin is achloromethylated polystyrene, which upon chemical reaction with theappropriate compound, provides a guanidine functionality carried by theion exchange resin. One of the preferred ion exchange resins useful inthe present invention is chloromethylated polystyrene, 1.06 meqchloride/g, 2% divinylbenzene (DVB). However, the particle size of theion exchange resin can vary widely, so long as the size range isgenerally fine enough to exhibit desirable loading and elution kineticsand yet large enough to (a) allow the solution to flow through the bedwithout binding or building up excess pressure: and (b) allow convenientscreening of the resin from the aqueous solution. Preferably, about a6-12 mesh size is employed. The loading of the water-insoluble ionexchange resins with the guanidine can vary widely. Generally, it willbe determined by the bed-volume characteristics of the particularwater-insoluble ion exchange resin. Typically, the flow rates throughthe ion exchange bed will be such as to assure effective absorption ontothe water-insoluble ion exchange resins.

After the water-insoluble ion exchange resin containing the guanidinereagent has been loaded with the precious metal values, the aqueouscyanide solution is separated from the ion exchange resin and theabsorbed precious metal values are eluted from the ion exchange resin.The suitable eluants as indicated are the same as the aqueous strippingsolutions employed in the liquid/liquid extraction process. The mostefficient and effective eluent is an aqueous solution of sodiumhydroxide having a pH above 11.

As indicated, both the liquid/liquid and liquid/solid extractionprocesses require reagents containing a guanidine functional group whichmay ideally be defined as: ##STR2## For use in the liquid/liquidextraction process there are water-insoluble guanidine compounds whichare soluble in water immiscible hydrocarbon solvents, and where preciousmetal salts are soluble therein, to the extent of at least 0.22% byweight. For use in the extraction process, the compounds also have a pKaof greater than 12 and preferably than 13. A discussion of basicstrengths of methylated guanidine and pKa values thereof can be seen in"The Basic Strength of Methylated Guanidines", S. J. Angyal and W. K.Worberton, pages 2492-2494 of J. Chem. Soc., 1951. In the liquid/solidextraction process, an ion exchange resin incorporates the guanidinefunctionality by chemical reaction with the guanidine compounds. Thus,the guanidine reagent suitable for use in the present extractionprocesses may be further illustrated by means of the idealized formula:##STR3## wherein R₁ through R₅ is selected from the group consisting ofH, an ion exchange resin backbone and aromatic and aliphatic groupshaving from 2-25 carbon atoms. THe guanidine compounds which areextraction reagents in the liquid/liquid system or which are chemicallyreacted with the ion exchange resin from the liquid/solid system arethose having a pKa at 25° C. greater than 12. Aromatic groups such asphenyl, tend to decrease the basicity to a level below a pKa of 12 andaccordingly not more than one of the R groups should be phenyl. The ionexchange resin may be bonded to the guanidine to any one of the nitrogenatoms such as at N" or N or N'. Further, any two of the nitrogen atomsmay form a cyclic structure with an R group, thus providing compounds ofthe formula: ##STR4## where R₆ is an aliphatic group having from 2-25carbon atoms. Further, any two of the nitrogen atoms such as the N, N'nitrogens, may form a cyclic structure containing additional nitrogenatoms such as in the formula: ##STR5## where Z is R¹ or NR'₂ where R' isH, aromatic or aliphatic hydrocarbon group having 1-25 carbon atoms.

In summary, the novel guanidines of this invention are those of theformula A. above in which:

(a) no more than one R group is aromatic such as phenyl;

(b) the R groups on nitrogen N and N' may be the same or different andat least one of the groups is an aliphatic hydrocarbon group containingat least 8 carbon atoms and preferably greater than 12;

(c) no more than 3 of the R groups R₁ through R₅ may be H;

(d) the sum of the carbon atoms in the R groups R₁ through R₅ is greaterthan 10, more desirably at least 16 carbon atoms and preferably greaterthan 20 carbon atoms, but if the sum is less than 25, at least one ofthe R groups is branched.

To further illustrate the various objects and advantages of the presentinvention, the following examples are provided. It is understood thattheir purpose is entirely illustrative and in no way intended to limitthe scope of the invention.

EXAMPLE 1

In this example the synthesis of symmetrical guanidine derivatives areillustrated. Using the procedure described below, there was prepared the

(A) di-n-octyl guanidine

(B) di-2-ethylhexyl guanidine, and

(C) di-tridecyl guanidine in which the tridecyl is a mixture of isomers

from the corresponding octylamine, 2-ethylhexylamine and tridecyl amine.

A solution of cyanogen bromide (5.3 g, 50 mmol) in octane (100 ml) wasadded to an octane solution (200 ml) with two equivalents of the amineat 0° C. After the addition, the mixture was heated to reflux at about100° C. for 8 hours. Workup with a sodium hydroxide solution gave thecrude products. The yields were 90-100%. The products were the evaluatedas extractants.

EXAMPLE II

In this example there is illustrated the preparation of an unsymmetricalguanidine derivative via a cyanamide intermediate.

There was prepared N-2-ethylhexylcyanamide by adding 2-ethylhexylamine(6.5g, 50 mmol) to a solution of octane (100 ml) containing cyanogenbromide (5.3g, 50 mmol) in the presence of one equivalent oftriethylamine at 0° C. The salt of triethylamine was removed byfiltration. The hydrogen chloride salt of a commercially availableamine, such as primary JMT C₁₂₋₁₄ amine of Rohm and Haas, was preparedby passing hydrogen chloride gas through an octane (200 ml) solution of50 mmol of the amines. The two octane solutions were then mixed, andheated to reflux at about 100° C. for 8 hours. The reaction then wasquenched by a solution of sodium hydroxide to give the unsymmetricalguanidine product, such as C₁₂₋₁₄, 2-ethylhexyl guanidine.

EXAMPLE III

In this example is illustrated the incorporation of the guanidinefunctionality into a water insoluble ion exchange resin.

There was synthesized N-(6-aminohexyl)-N'-butylguanidine via thecyanamide intermediate, N-(6-aminohexyl)-cyanamide. The intermediate wasprepared by adding one equivalent of cyanogen bromide to a solution of1.6-diaminohexane (one equivalent) in octane at 0° C. Then to thissolution was added n-butylamine (five equivalents). Workup with a sodiumhydroxide solution gave the desired guanidine with an amino group on theother end, which was reacted further with chloromethylated polystyrene.Chloromethylated polystyrene (1.06 meq/g, 2% DVB, 200-400 mesh) wastreated with an excess amount (4 equivalents of guanidine per equivalentof chloride) of this guanidine derivative in a mixture ofdimethylformamide and tetrahydrofuran, and heated to reflux for 24hours. This modified resin then was washed with a sodium hydroxidesolution, followed by water and ethanol. Finally it was dried undervacuum.

EXAMPLE IV

(a). N,N'-Di-n-octylguanidine prepared in Example I was then evaluatedas an extractant for gold, silver and copper from synthetic cyanidesolutions containing approximately 10 ppm of gold, silver or copper inthe presence of sodium cyanide (500 ppm) between pH=7 and pH=11.20.Organic solutions with 0.050 M of the extractant in 10% tridecanol (TDA)and 90% xylene were contacted with equal volumes of the cyanidesolutions for 5 minutes, different pH values adjusted by either asulfuric acid solution or sodium hydroxide solution. Subsequent analysisof the aqueous solutions for metal content indicated that the extractionpercentage of gold, silver and copper was quantitative within this pHrange.

(b). The other two guanidines in Example I were also evaluated asextractants for gold, silver and copper from cyanide solutions by thesame procedure. Kerosene was used as a diluent, butN,N'-bis(2-ethylhexyl)guanidine required 10% TDA as a modifier.Subsequent analysis of the aqueous phase generated the data in Table Ibelow to obtain pH isotherms.

                  TABLE I                                                         ______________________________________                                        Extraction Percentage of Gold, Silver and Copper vs. pH                       ______________________________________                                        1. 0.050 M of N,N'--bis(trideyl)guanidine                                     pH         % of Au Extraction                                                 ______________________________________                                         9.85      100%                                                               10.90      100%                                                               12.20       90%                                                               ______________________________________                                        pH         % of Ag Extraction                                                 ______________________________________                                        10.10      100%                                                               11.30       95%                                                               12.00       64%                                                               ______________________________________                                        2. 0.010 M of N,N'--bis(tridecyl)guanidine                                    pH         % of Au Extraction                                                 ______________________________________                                         9.10      100%                                                               10.05      97%                                                                11.10      82%                                                                12.30      19%                                                                ______________________________________                                        pH         % of Ag Extraction                                                 ______________________________________                                         9.55      93%                                                                10.70      46%                                                                11.65       9%                                                                12.20       0%                                                                ______________________________________                                        pH         % of Cu Extraction                                                 ______________________________________                                         9.30      100%                                                               10.20       89%                                                               11.20       23%                                                               12.05       6%                                                                ______________________________________                                        3. 0.050 M of N,N'--bis(2-ethylhexyl)guanidine                                pH         % of Au Extraction                                                 ______________________________________                                         9.95      100%                                                               10.60      100%                                                               12.20       95%                                                               ______________________________________                                        pH         % of Ag Extraction                                                 ______________________________________                                        10.35      100%                                                               11.70       76%                                                               12.30       35%                                                               ______________________________________                                        pH         % of Cu Extraction                                                 ______________________________________                                        10.05      100%                                                               10.55      99%                                                                12.00      48%                                                                ______________________________________                                        4. 0.010 M of N,N'--bis(2-ethylhexyl)guanidine                                pH         % of Au Extraction                                                 ______________________________________                                         8.75      100%                                                               10.40      92%                                                                11.80      34%                                                                12.10      30%                                                                ______________________________________                                        pH         % of Ag Extraction                                                 ______________________________________                                         9.50      56%                                                                10.00      34%                                                                10.75      21%                                                                11.85       4%                                                                ______________________________________                                        pH         % of Cu Extraction                                                 ______________________________________                                         9.50      20%                                                                10.30       8%                                                                11.25       6%                                                                ______________________________________                                    

EXAMPLE V

In order to illustrate the selectivity, a solution (15 ml) of kerosenewith 0.010 M concentration of N,N'-bis-(2-ethylhexyl)guanidine wascontacted by an aqueous solution (15 ml) containing approximately 8 ppmof gold, silver and copper at pH=10.30, and pH=10.90. The extractionpercentage of each metal ion is shown in Table III below.

                  TABLE III                                                       ______________________________________                                        pH          Au         Ag         Cu                                          ______________________________________                                        10.30       95%        68%        49%                                         10.90       89%        49%         5%                                         ______________________________________                                    

These competitive selectivity experiments show a preference ofgold>silver>copper.

EXAMPLE VI

This example illustrates that gold can be recovered from the loadedorganic solutions by stripping with 10% sodium hydroxide solutions. Bycontacting the organic solution containing the gold values with thestripping solution with an O/A of 1, the gold was quantatively recoveredfrom the loaded organic solutions (0.050 M and 0.010 M of :heextractants). Gold can be concentrated by simply changing the O/A ratioin both extraction and stripping. For stripping, gold concentration wasincreased from 16.9 ppm to 64 ppm (93% recovery) by stripping an organicsolution (0.025 M of N,N'-bis(2-ethylhexyl)guanidine) with an O/A of 4.

The following Table IV illustrates the results of the stripping tests.

                  TABLE IV                                                        ______________________________________                                        R               0/A    Recovery Percentage                                    ______________________________________                                        0.050 M                                                                              tridecyl     1      100%                                               0.010 M                                                                              tridecyl     1      100%                                               0.010 M                                                                              tridecyl     5       95%                                               0.050 M                                                                              2-ethylhexyl 1      100%                                               0.010 M                                                                              2-ethylhexyl 1      100%                                               0.010 M                                                                              2-ethylhexyl 5       93%                                               0.025 M                                                                              2-ethylhexyl 4       93%                                               ______________________________________                                    

EXAMPLE VII

In this example extraction was conducted using the resin of Example III.The modified resin (1 g) was contacted overnight with an aqueoussolution (20 ml) containing 10 ppm of gold in the presence of 500 ppm ofcyanide. Then, the aqueous phase was filtered. Gold was quantativelyextracted at pH of 10.65. In a control experiment by use of theunmodified resin, gold was not extracted at all.

EXAMPLE VIII

The gold was recovered from the loaded 1 gram of modified resin fromExample VII above by stripping with 20 ml of a 10% sodium hydroxidesolution. The gold was recovered in an amount of 40%.

We claim:
 1. A process for the recovery of a precious metal from anaqueous alkaline cyanide solution containing said precious metalcomprising:(1) contacting said aqueous solution containing said preciousmetal with an ion exchange resin having a functionality consistingessentially of guanidine functionality wherein said precious metal valueis extracted from said aqueous solution, wherein said ion exchange resincontaining said quanidine functionality has a pKa at 25° C. greater than12 and has the formula: ##STR6## where one of the R groups R₁ through R₅is an ion exchange resin backbone and the remaining R groups areselected from the group consisting of H and aromatic and aliphatichydrocarbon groups having up to 25 carbon atoms; (2) separating saidaqueous solution, now metal barren, from said ion exchange resin havingsaid guanidine functionality; and (3) recovering the precious metalvalue from said ion exchange resin by stripping said precious metalvalue from said ion exchange resin by contacting said ion exchange resinwith an aqueous alkaline solution.
 2. A process as defined in claim 1wherein said resin is a polystyrene resin.
 3. A process as defined inclaim 1 wherein said precious metal is gold or silver.
 4. A process asdefined in claim 1 in which said aqueous alkaline solution for strippingsaid precious metal value from said ion exchange resin has a pH above11.
 5. A process as defined in claim 4 wherein said aqueous alkalinestripping solution is NaOH.
 6. A process as defined in claim 1 whereinthe R group which is the ion exchange resin backbone is a polystyreneresin, two of the R groups are H and the remaining two R groups, whichmay be the same or different, are alkyl groups selected from the groupconsisting of n-octyl, 2-ethylhexyl, butyl, hexyl and tridecyl.
 7. Aprocess as defined in claim 6 wherein one of the two remaining R groupsis butyl and the other in hexyl.
 8. A precious metal cyanide complex ofa guanidine compound having a pKa at 25° C. greater than 12 and havingthe formula: ##STR7## where one of the R groups R₁ through R₅ is an ionexchange resin backbone and the remaining R groups are selected from thegroup consisting of H and aromatic and aliphatic hydrocarbon groupshaving up to 25 carbon atoms.
 9. A precious metal cyanide complex asdefined in claim 8 wherein said precious metal is selected from thegroup consisting of silver and gold.
 10. A precious metal cyanidecomplex is defined in claim 8 in which said ion exchange resin backboneis a polystyrene resin.
 11. A precious metal cyanide complex as definedin claim 10 in which said precious metal is gold.
 12. A precious metalcyanide complex as defined in claim 8 wherein the R group which is theion exchange resin backbone is a polystyrene resin, two of the R groupsare H and the remaining two R groups, which may be the same ordifferent, are alkyl groups selected from the group consisting ofn-octyl, 2-ethylhexyl, butyl, hexyl and tridecyl.
 13. A precious metalcyanide complex as defined in claim 12 in which said precious metal isgold.
 14. A precious metal cyanide complex as defined in claim 13 inwhich one of the two remaining R groups is butyl and the other is hexyl.